The #circadian rhythm controls a wide range of functions in the human body and is required for optimal health. Disruption of the circadian rhythm can produce #inflammation and initiate or aggravate chronic diseases. The modern lifestyle involves long indoor hours under artificial lighting conditions as well as eating, working, and sleeping at irregular times, which can disrupt the circadian rhythm and lead to poor #health outcomes. Seasonal solar variations, the sunspot cycle and anthropogenic electromagnetic fields can also influence biological rhythms. The possible mechanisms underlying these effects are discussed, which include resonance, radical-pair formation in retina cryptochromes, ion cyclotron resonance, and interference, ultimately leading to variations in melatonin and cortisol. Intracellular water, which represents a coherent, ordered phase that is sensitive to infrared light and electromagnetic fields, may also respond to solar variations and man-made electromagnetic fields. We describe here various factors and underlying mechanisms that affect the regulation of biological rhythms, with the aim of providing practical measures to improve human health.
Biological rhythms involve changes in living organisms that vary as a function of daylight, seasons, and the solar cycle. The most studied biological rhythm is the 24-hour circadian clock which is observed in most living organisms [1]. Light from the sun is the main Zeitgeber that entrains the circadian rhythm and sleep/wake cycle to a period of 24 hours, corresponding to one full rotation of the Earth on its axis relative to the Sun [1]. Under controlled light and temperature conditions, the circadian rhythm persists but its period becomes shorter or longer than 24 hours [2]. The circadian rhythm plays an important role in maintaining the regularity of daily body functions and ensures their synchronization with environmental changes―especially those of the sun. Moreover, the circadian rhythm is thought to provide advantages for living organisms by synchronizing body functions according to time of the day and favoring optimal use of the limited resources and energy available. Indeed, extrinsic controls of nutrient consumption, ambulation, sexual behavior, and rest or sleep are likely to increase fitness outcomes relative to ad hoc decision-making by the organism.
The circadian clock controls most physiological functions including sleep/wake, fasting/eating, and anabolic/catabolic cycles, as well as variations in body temperature, hormones, and immune functions [3]. Disruption of the circadian rhythm by sleep deprivation, night shift work, and jet lag is associated with sub-optimal body functions, inflammation, and various symptoms (e.g., fatigue, fever, discomfort, pain, headache). Given the importance of the circadian rhythm, it is not surprising that chronic disruption is associated with the development of some of the most prevalent diseases and causes of death such as type 2 diabetes, obesity, cardiovascular disease, cancer, mood disorders, and neurological diseases [4]. A bidirectional relationship exists between the circadian rhythm and disease in which circadian disruption increases disease severity while many disease-causing factors such as stress and substance use can disrupt circadian rhythmicity [4].
In order to maintain circadian rhythmicity, the suprachiasmatic nucleus (SCN) of the hypothalamus must be reset by daily light that activates photoreceptors in the retina (Fig. 1). Neuronal signals from this “master clock” are then relayed to several peripheral clocks that regulate the function of other organs such as the heart, liver, and lungs via the autonomic nervous system and hormones such as glucocorticoids, catecholamines, and melatonin. The circadian rhythm involves intracellular transcription-translation feedback loops in which rhythmic expression of clock genes regulate the expression of downstream gene products, followed by repression of their own expression (Fig. 1). In most vertebrates, the transcription factors CLOCK and BMAL1 modulate the expression of period (Per1, 2, 3) and cryptochrome (Cry1, 2) genes, which in turn repress gene expression to allow a new cycle [5]. This oscillating molecular clock regulates thousands of genes in a tissue-specific manner (Fig. 1).
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